A critical step in disk file assembly is the merge operation in which read/write heads are merged with the disk stack after first being mounted in a head positioning actuator. Conventionally, this operation takes place after the mounting of the disk stack assembly and the actuator assembly in a common base casting. The merge operation requires the pre-positioning of the heads beyond the disk stack periphery with the resilient head suspensions collapsed. This allows them to be moved subsequently into the inter disk spaces where the suspensions are released, thus allowing the heads to contact the disks. The risk of damage to the delicate heads and disk surfaces is very high during this operation.
Where the base casting does not restrict access, elaborate tooling may be employed to control the merge operations carefully. One such system is shown in GB 2196924A for a disk file having a relatively flat base casting and employing rotary actuators.
In some disk files, base castings with relatively high side walls surrounding the disks restrict access to the heads and disks at the critical point of merger during assembly. Such a disk file is shown in U.S. Pat. No. 4,661,729. Although not described in that patent, a special tool, as described in an article entitled "Head Load/Unload Mechanisms" by C. M. Lacey and R. R. Newman (IBM TDB Vol. 31 No.2 July 1988 p.231), is needed to effect head disk merge in such a disk file.
A recent trend in disk file design has been the provision of all round castings which are either split like a clam-shell or take the form of an open box, the open side of which is spanned by the disk stack in its axial direction. The latter type of structure is very strong. This, together with its inherent symmetry, allows greater track densities to be achieved than is otherwise the case with a conventional head positioning servo system. One example of such file is described in EP 0222932 Al. Head/disk merge is not discussed in this document but, in practice, could only be achieved by designing the base casting with cut away sides leaving the disk stack supported by only upper and lower flanges. Limited access to the heads and disks at the point of merging is then possible through the cut away side areas and the disk stack.
Other examples of box like base castings are in JP 56-169271A and GE 0151259A. The former document (abstract) does not show the actuator assembly or how it is merged with the disks but the relatively low number of disks in the stack may have enabled access to the heads through the inter-disk spaces. The latter document shows a swinging arm rotary actuator mounted on the same side of the box casting as the disk stack spindle which affords greater accessibility to the head/disk interface. This option is not available where a linear actuator is employed . Also, despite having some advantages, a swing arm rotary actuator is inherently asymmetrical as compared with a linear actuator mounted radially on a plane of symmetry of the disk stack assembly and positioning loops employing rotary actuators have a lower bandwidth.